Helical antenna system



Feb. 12, 1957 w. slcHAK ET Al.

HELICAL ANTENNA SYSTEM Filed April 29, 195s MNY A MIAS@ A Nm u Nm. w A un N. xv R H ANN ma, mN

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Unite 2,781,514 rrEL'ICAL ANTENNA SYSTEM Application April 29, 1953, Serial No. 351,852

11 Claims. (Cl. 343895) This invention relates to helical antennas and more particularly to a helical antenna and tuning system for the same, capable of operating `over an extremely wide frequency range. r

ln many applications, practical considerations require that the height of an antenna associated with a radio transmitter be very small when compared to the desired operating wavelength. Such conditions often arise in aerial radio system applications where the antenna must not only be small in physical size, have proper aerodynamic characteristics but must also be capable of operating over a wide frequency range. One common type of antenna utilized a whip antenna which must be supported and fed through a base insulator. When such an antenna is short in length when compared to the operating wavelength there is a large voltage developed across the base insulator thereby introducing an appreciable loss of power in the base insulator. The helical antenna system as proposed allows an increase in the radiation resistance of 50% or more to be realized over a whip antenna for the same physical height but even more important is the fact tha-t it allows the base insulator to be eliminated.

Heretofore helical antennas have been designed for operation over a wide frequency range. It is well known that such antennas are designed for quarter wave resonance at their lowest operating frequency and in order to operate over a wide frequency range it is necessary to electrically shorten the antenna. Various methods of electrically shortening helical antennas have been proposed such as means for physically decreasing the height of the radiator extending above a ground screen. Such an electrical shortening of the helical antenna by decreasing the height of the` radiator extending above the ground plane has proven unsatisfactory and undesirable from the electrical standpoint since the antenna Q under such adjustment remains undesirably high over the entire frequency band. In addition, it has been usual in the past to vary the point of input coupling on the antenna. It has been found that providing a variable point of coupling creates many mechanical problems which in practice are difficult to solve.

One of the objects of this invention, therefore, is to provide a tuned helical antenna system having an optimum average bandwidth over a large frequency range.-

Another object of this invention is to provide a tuned helical antenna system of relative high efficiency wherein the physical height of the radiator extended above the ground plane is maintained constant.

A further object of this invention is to provide a 'tuned helical antenna system which is mechanically easy and simple to construct.

A feature of this invention is the use of a helically shaped radiator extending above a ground plane conductor and having its base connected thereto. The center conductor of a coaxial feedline is connected to the helix in a manner to form a shunt feed. An adjustable shorting device varies lthe electrical length of the radi- States Patent grice ator extending 4above the ground plane without varying the physical height of the antenna. The input coupling tap connect-ion formed by the center conductor remains fixed with respect to the leading edge of the shorting device as the shorting device Iis adjusted to vary the operating frequency of the antenna.

The above-mentioned and `other features yand objects of this invention will become more apparent by reference to the following description taken in conju-nction with lthe accompanying drawings, in which:

Fig. l is a longitudinal cross ysectional view of one embodiment of a continuously tuned helical antenna system in 'accordance with the principles of this invention; and,

`Fig. 2 is a longitudinal cross `sectional view of another embodiment of the invention wherein the helical antenna system is capable of being tuned in finite steps.

Referring t-o Fig. 1 of the drawing, a continuously tuned helical :antenna system is shown comprising a supporting structure 1 in 'the form of a cylinder composed of a low loss dielectric material such as fiberglass. A conductive element 2 is wound in the shape of a helical radiator 3 supported within and coaxially of the supporting cylinder 1. The helix 3 is designed to be resonant at the lowest Voperating frequency of the antenna system. The supporting cylinder 1 and helix 3 are supported by a ground plane conductor or screen 4. The lower end or base 5 `of the helix 3 is grounded to the plane conductor 4. An electrical shorting device 6 extends through the ground plane 4 into and coaxially of the helix 3. The shorting device 6 comprises a threaded conductive cylinder 7 terminated .at its upper end by la plurali-ty of shorting fingers 8. To insure positive shorting of the portion of the helix 3 below fingers 8 the circumference `of the shorting fingers 8 is made equal to one complete turn of the conductive element 2 forming helix 3. The threads 9 lon the outer circumference of 4the shorting cylinder 7 have an outer diameter equal to the inner diameter -of the helix 3 and the pitch of the threads 9 is equal to the pitch of helix 3. The design of the threads 9 insures positive contact between the shorting device 6 and the helix 3. The threaded cylinder 7 is supported by `structure 10 comprising a conductive tube or nut connected to the ground plane conductor 4 and meshed with the threads 9 of device 6. Carried by and extending inside and coaxially of the cylinder 7 is a radio frequency feedline 11 comprising an inner conductor 12 and an outer conductor 13 separated by dielectric material 14. The outer conductor 13 is in electrical contact with and grounded by the cylinder 7. The inner conductor 12 extends beyond the upper limit of the shorting device 6 and fingers 8, terminating in a constant pressure wiping contact 15 for engagement with the helical conductor 2. The contact 15 is maintained a fixed distance from the shorting fingers 8. The lower end -of the coaxial cable 11 terminates in a rotating joint 16. A feed-line 17 couples energy from a transmitter through joint 16 to the coaxial feedline 11 and thence to the antenna 3 through shunt feed 15.

At its lowest operating frequency the maximum height of the helix 3 must radiate or in other words the shorting fingers 8 -of device 6 lare in the lowest position contacting the conductive element -2 near the base 5 of helix 3. In order to tune the helical antenna for optimum efliciency at a higher frequency the shorting cylinder 7 together with the coaxial cable 11 are rotated causing the shorting fingers 8 to advance up the helix 3. As the shorting device 6 is rotated and advanced up the antenna the shunt feed 15 also advances maintaining a constant distance above the shorting ngers 8 which effectively shorts out all turns of the helix between the fingers 8 and the base 5 of the helix 3. As the operating frequency of the antenna is increased the bandwidth of the antenna system is increased.

Referring to Fig. 2 of 'the drawing, another helical antenna system in accordance with the principles of this invention isshown which is ycapable of being tuned in iinite steps andycomprises ,a supporting structure 18 in the form of -a low loss ldielectric tube or cylinder. A conductive element 19 is wound in the shape `of a helical radiator 20 around the outer circumference of tube The 4helical antenna 20 is designed to lbe resonant at the lowest operating frequency of theantenna system. The supporting tube 1? and helixZtl are mounted on a portion 21 of ground plane conductor 2 2. AStructuralsupport for portion 2 1 is provided by asupporting post 23 ,which is coupled to the tube land connected on its o-ther end to `supporting braces 24 and,2,5. The braces 24V and 25 are connected to theground plane conductor 22. The lower end or brace 27 of the' helix 20 is grounded .via 4the metallic base 21, contacts 32, cylinder 29 and contact fingers 31. An electrical shorting Y device 28 extends through the ground plane 22 coaxially ofthe helix 20. The shorting device 23 comprises a conductive cylinder 29 terminated at its upper end by a plurality of shorting fingers 30 engaging the helix Ztl. To insure positive shorting of the portion of the.helix 2,0 below the fingers 30 spring contacts 31 are provided between the ground plane 22 `and cylinderl 29 and contact is maintained between fthe metallic supporting structure 21Y and cylinder 29 by means of contacts V32. A coaxial feedline 33 is rigidly coupled to the cylinder 29 by meansgof conductive `brace 38 to which the outer conductor 36 of the feedline is connected. ,A brace 34- i-s terminated ina wiping contact 35 to couple vthe inner conductor l37 of coaxial feedline 33 as a shunt feedto the helix 20. The contact 35 is maintained a fixed distance from the jshorting fingers 30. The feedline 33 couples energy fromV a transmitter (not shown) through contact 3,5 Vto the ,helix 20.

At its lowest operating frequency the maximum height of the antenna 20 must radiategor in other words'vthe shorting fingers 30 of device 28 are in the lowest position contacting the conductive element 19 near vthe base 27 of the helix 20. In order to tune thel helical antenna for optimum efliciency. at a higher frequency, the shorting cylinder 29 -together with coaxial feedline 33 are moved up ythe height of the cylinder causing shorting lingers 30 to electrically short out the lower turns of the helix `2t). As the shorting device28 advances up `the Iheightfof the antenna `the shun feed 35 is maintained a constant distance from the shorting fingers 30, thusV effectively shorting out all turnsyof the helixy 2G between the ting'ersfS l and the base 27. -As the operatingfrequency of the anin the accompanying claims.

We claim: l.,A radio antennawsystem comprising a conductive element disposed in fthe form of a helical radiator, means to couple radio` frequency energy to said helical radiator, means adjustable to short out portionsfof saidhelical `radiator to thereby change the effective electrical length of said radiator forresonance at different radio frequencles, andme-ans to maintain the positionof said. coupling Vmeans ay given distance from said adjustable Ashorting means.

'l 2..A radio-antennasystemcomprising a groundl plane conduct-or, a conductive' element having one end coupled to.- saidground plane conductor disposed in the'form of -a helical radiator extending a given height above said ground plane conductor, vadjustable means coupled to said ground plane conductor to effectively short out selected portions of said helical radiator in order to adjust the effective electrical length of said radiator for resonance at different radio frequencies and means to couple radio frequency energy to said radiator at a predetermined distance above said adjustable shorting means.

3. A radio antenna system comprising a conductive element disposed in the form of a helical radiator extending a given distance above ground, means to couple energyA from a radio frequency source at desired frequencies within a relatively broad frequency range to said helical radiator, said coupling means including a coaxial transmission linehaving itsy outer conductor grounded, a coupling element coupled to the inner conductor of said line and to said helical radiator, means to adjust the effective electrical length of said radiator to render said helical radiator resonant at a desired frequency, and means to vary said coupling element relative to the turns of said helical radiator to maintain said coupling element in a predetermined relationship to said adjusting means.

"4. Al radio antenna system comprising a dielectric supporting structure, a conductive element disposed in the Vform 'of' a helix supported by said structure, means to adjustith'eeffective electrical length of said radiator to cause aiportion of said helical radiator to resonate at a desired l frequency, and meansto couple energy at said radiofrequency to said helical radiator at a predetermined distar'i'ce'v from said adjustable means.

A'5j-A radio antenna system comprising a ground plane conductor, a dielectric supporting structure, a conductive velement having one end Vcoupled to said ground plane 'conductor disposed in the form of a helical radiator supportedby saiddielectric structure extending above said ground-plane conductor, adjustable means coupled to said 'ground'plane conductor' to short out portions of said adjustable means having an outer conductor grounded and an 'inner conductor with one end thereof coupled to a source of radio frequency energy, and means to cause the second end of said inner conductor to couple energy to a Aportion of said helix at a given distance from said adjustable shorting means.

i v6. 'A radio 'antenna system comprising a ground plane conductor, a dielectric supporting structure of cylindrical shape'carried by said ground plane conductor, a conduca tive element having .one end coupled to said ground plane u conductor disposed 'in the form of a helical radiator around saidfsupporting structure, a conductive cylinder grounded to said ground plane conductor and adjustably movablyalong the axis of said helical radiator, a plurality of contacts mounted on one end of said cylinder to electrically couple said conductive cylinder to said helical radiator to shortcut portions of said helical radiator to thereby 'change 1the effective electrical length of said fil) radiator for resonance, a coaxial transmission line carried by said conductive cylinder having an outer conductor coupled to said ground plane conductor and an inner conductor with oneend thereof coupled to a source of v radio frequency energy, andmeans to cause the second end of said inner conductor to couple energy to a portion of said helix at a .given distance above said cylinder mounted contacts.

7. A radio antenna system comprising a groundplane conductor, a dielectric supporting structure of hollow cylindrical shape, a conductive element having one end coupled to said ground plane conductor disposed in the formof a helical -radiator supported by and inside said dielectric structure extending above said vground plane conductor, vadjustablefrne'ans coupled to said ground plane fcnondctor'to shortcut portions of said helical radiator to Athereby change thefeffective electrical lengths of said 5 radiator for resonance, a coaxial transmission line carried by said adjustable means having an outer conductor grounded and an inner conductor with one end thereof coupled to a source of radio frequency energy, and means to cause the second end of said inner conductor to couple energy to a portion of said helix at a given distance above said adjustable shorting means.

8. A system according to claim 7 which further includes a rotating joint, said coaxial transmission line being coupled to said radio frequency energy through said rotating joint.

9. A system according to claim 7 wherein said adjustable shorting means comprises a threaded conductive cylinder having an outside thread diameter equal to the inner diameter of said helix.

10. A system according to claim 9 wherein said conductive cylinder is terminated by a plurality of conductive spring fingers, said plurality of spring ingers having a circumference equal to the circumference of said helix whereby one complete turn of said helical radiator may be shorted to said conductive cylinder.

1l. A device according to claim 9 wherein the pitch of said threads on said conductive cylinder is substantially equal to the pitch of said helical radiator.

References Cited in the tile of this patent UNITED STATES PATENTS 1,517,570 Mauborgne et al Dec. 2, 1924 2,415,736 Flood et al Feb. 11, 1947 FOREIGN PATENTS 576,23() France Aug. 13, 1924 

